1. Field of the Invention
The present invention relates to an air bag system as a supplemental restraint system to a seat belt apparatus for a vehicle, and, more particularly, to an air bag system supplementary to a seat belt apparatus which inflates and expands an air bag with high and low levels of inflation pressure according to variable threshold values.
2. Description of the Related Art
Air bag systems are well known as a supplementary restraint system to seat belt apparatuses for vehicles in the art. Such an air bag system includes a plurality of crash sensors, an air bag installed in the inside of a steering wheel, and a control unit that controls inflation and expansion of the air bag in response to at least an output signal from any one of the crash sensors. The crash sensor may be an acceleration sensor that provides an electrical output signal having a value related to the energy of impact of a crash. The control unit evaluates incoming electrical signals to control inflation and expansion of the air bag upon an occurrence of a crash. An actuation circuit includes a squib to fire an inflammable fluid to generate inflation gas which is discharged into the air bag and inflates and expands it. Upon an occurrence of a crash, the air bag is instantaneously inflated and expanded to protect a passenger from hitting the steering wheel or front part of the passenger compartment.
The prior art air bag systems have recognized that it is not always desirable to inflate the air bag with 100% of the gas generated by firing the fluid. In recent years, there has been proposed air bag systems of a type in which the air bag is inflated and expanded with different inflation pressure, namely a low level of inflation pressure and a high level of inflation pressure according to whether the passenger is fastened by a seat belt or not.
The shock absorbing capacity of the air bag changes according to whether the passenger is fastened by the seat belt and/or the levels of inflation pressure with which the air bag is inflated and expanded. As shown in FIG. 3 regarding the shock absorbing capacity performance of an air bag with respect to vehicle crush speed, characteristic curves "a," "b" and "c" show the shock absorbing performance when the air bag is not inflated, when the air bag is inflated with a low level of inflation pressure and when the air bag is inflated with a high level of inflation pressure, respectively, while the passenger is not fastened by the seat belt. The characteristic curves "d," "e" and "f" show the shock absorbing performance when the air bag is not inflated, when the air bag is inflated with the low level of inflation pressure and when the air bag is inflated with the high level of inflation pressure, respectively, while the passenger is fastened by the seat belt. As apparent from FIG. 3, the shock absorbing performance of the air bag against the passenger is different among conditions of inflation and expansion of the air bag. Further, as shown by double-dotted line in FIGS. 4 and 5, the shock absorbing performance decreases with an increase in vehicle speed at an occurrence of a crash. This is because the higher the vehicle crash speed raises, the more the kinetic energy of the passenger has. Japanese Patent No. 2,507,796 discloses an air bag system of a type which includes two discharge valves and a valve controller. Both discharge valves adjust the internal gas pressure in the air bag during the inflation of the air bag. The valve controller controls the discharge valves according to whether the passenger is fastened by a seat belt, the weight of the passenger, the vehicle speed and so on. Specifically, the first discharge valve is controlled to maintain a first level of internal pressure of the air bag at which the air bag attains a first effective restraint state. The second discharge valve is controlled to maintain a second level of internal pressure of the air bag at which the air bag attains a second effective restraint state while the inflation gas pressure rises. When the air bag internal pressure reaches the first level of internal pressure during inflation and expansion of the air bag, the first valve discharges the inflation gas to maintain the first level of internal gas pressure. While the inflation gas pressure sharply rises after the air bag has attained the first effective restraint state, the second valve discharges the inflation gas pressure to maintain the second level of internal pressure. The air bag system adjusts the first and second levels of internal pressure to appropriate levels according to whether the passenger is fastened by the seat belt or not, the weight of the passenger, the vehicle speed to provide appropriately effective protection of the passenger during an occurrence of a crash.
Japanese Unexamined Patent Publication No.7-277123 discloses an air bag system equipped with an inflator incorporating an electrical heater which is effected while the seat belt is buckled up. The heater remains effective while the seat belt is unbuckled to heat an inflation gas so as to heat the inflation gas and rise its pressure with which the air bag is inflated when a crash is happened but switched off while the seat belt is buckled up.
Some of improved air bag systems are equipped with what is called a composite or multiple type of inflator unit having at least two inflators which can be actuated independently from each other. Both inflators discharge the same volume of an inflation gas into an air bag. Either one of the two inflators is actuated to inflate and expand the air bag with a low level of inflation gas pressure when a light crash is happened, or both inflators are actuated to inflate and expand the air bag with high level of inflation gas pressure when a crash is heavy.
None of air bag systems equipped with the multiple type of inflator unit varies a threshold value for actuating either one or both of the inflators according to whether the seat belt is buckled up and vehicle running states (which includes a vehicle speed, a steering angle and so on). For instance, the air bag system may be controlled such that the air bag is inflated with a low level of inflation pressure always at an occurrence of a crash and is subsequently inflated with a high level of inflation pressure. In such a case, while there is no problem when the crash velocity of a vehicle is relatively low, nevertheless, the impact absorbing performance will decrease to some extent when the crash velocity of a vehicle is relatively high.